Embryology of the GI tract

Development of the glands

1. How the epithelial tissue cells proliferate and penetrate the connective tissue:

A. Endocrine glands without contact with the surface is formed , notice the amount of cells

B. Exocrine glands maintain contact with the surface by ducts and are formed.

Development of the glands

Study the development of salivary glands and how the duct and acini of the parotid gland are derived from ectoderm

Submandibular and sublingual glands are derived from endoderm

Development of the mouth

Study how the mouth is formed from 2 sources and notice that the hard palate, sides of the mouth, lips and enamel of the teeth developed from ectoderm

Study how the floor of the mouth, tongue and soft palate and palatoglossal and palatopgaryngeal folds are developed from endoderm

Development of the mouth

Study the development of the tongue and notice:

1. Tuberculum impar (endoderm)

2. Lateral lingual swelling (developed from the first pharyngeal arch)

3. Second median swelling (copula)

4. The third pharyngeal arch from the posterior third of the tongue

5. The sulcus terminalis represent the interval between the first and third pharyngeal arches.

Development of the pharynx

The student should study the development of the pharynx in the neck from the endoderm of the foregut.

How the endoderm is separated from the surface ectoderm by a layer of mesenchyme

How the mesenchyme on each side becomes split up into (5-6) pharyngeal arches

Development of the anterior abdominal wall and abdominal muscles

1. Development of ectoderm

2. Development of endoderm

3. Segmentation of the mesoderm as the lateral mesoderm split into somatic and splanchnic mesoderm

4. How the anterior abdominal muscles is developed from the somatopleuric mesoderm

5. How the rectus abdominis is developed inside the rectus sheath and how the linea alba in midline is developed

Development of the

umbilical cord and the umbilicus

1. How the body stalk and yolk sac with their blood vessels form the tubular umbilical cord

2. Notice the loose connective tissue in the umbilical cord contains vitteline duct, remains of yolk sac , remains of alluntois, umbilicak vessels (arteries and veins)

Clinical points

Study the following :

1. The Meckel’s diverticulum

2. The persistence of vitellointestinal duct which causes umbilical fecal fistula

Development of the primitive gut which is divided into pharyngeal gut (pharynx)

1. Foregut: extends to liver bud

2. Midgut : extends from liver bud to lateral third of transverse colon

3. Hindgut: extends from lateral third of the T-colon to the cloacael membrane

Development of the esophagus

Study how it is developed as a part of foregut :

1. First it is short and then elongate rapidly downwards

2. Study the clinical abnormalities (fistula)

Development of the stomach

1. Study how the stomach from the greater and lesser curvature as a result of rapid growth of dorsal mesentery and slow growth of ventral mesentery

2. As a result of rotation of the stomach of the right side it forms the anterior and posterior surfaces and the right and left vagi become anterior and posterior

3. As a result of rotation the mesenteries form the omenta (lesser and greater omentum) and ligaments of the stomach

4. The dorsal mesentery forms the ligaments of the stomach and mesenteries of small and large intestines as greater omentum.

5. The ventral mesenteries forms the liver ligaments

Note: during 6 weeks of development the capacity of abdominal capacity is greatly reduced due to great enlargement of liver and kidneys so physiological herniation of mid gut results

Development of greater and lesser sac of peritoneum

1. Observe the extensive growth of the right lobe of the liver which pulls the ventral mesentery to the right side

2. Observe the rotation of the stomach and the duodenum

3. Observe the right free border of ventral mesentery becomes the right border of the lesser omentum

4. Observe the greater sac the remaining part of the peritoneal cavity

5. Observe the epiploic foramen

6. Observe the formation of the greater omentum as a result of rapid and extensive growth of the dorsal mesenteries

Development of the liver and bile duct

1. Observe the liver bud at the distal end of foregut. It is entodermal cells lies at the apex of the loop of developing duodenum (mid of the second part of duodenum)

2. Observe that the liver bud grows into mass of splanchnic mesoderm called (septum transversum)

3. Observe that the end of liver bud divides into right and left

4. Endodermal cells grow into vascular mesoderm which forms liver cords and liver sinusoids

5. Formation the ducts by canalization (hepatic ducts)

Development of gallbladder and cystic duct

1. Observe the end of hepatic bud expands forming gallbladder and the remains duct forms cystic duct

2. Billiary atresia failure of bile duct formation

3. Observe some clinical abnormalities in liver and gall bladder

Development of duodenum

1. How it is formed from the most caudal part of foregut and cephalic end of midgut

2. Observe the dorsal and ventral mesenteries and the rotation of the stomach which forced the duodenum to rotate to the right side so parts of the duodenum is formed.

3. Observe the disappearance of peritoneum behind the duodenum so it is retroperitoneum

4. Observe the site of ligament of tritz which fixes the terminal part of duodenum

Development of the pancreas

1. Observe that the development from the dorsal and ventral bud of entodermal cells that arise from the foregut

2. Observe that as a results of rotation of the stomach and duodenum, with the rapid growth of the left side of duodenum the ventral bud becoming into contact with the dorsal bud and fusion occurs.

3. Observe the development of islets of langerhans from small buds from developing duct these from group of cells secretes insulin as glucagon (at 5th mouth)

4. Ventral pancreatic bud formed head of pancreas (inferior

part) uncinate process

5. Dorsal pancreatic bud formed a. Superior part of head b. Neck c. Body d. Tail of pancreas

5. some of pancreatic abnormalities : a. Annular pancreas b. Duodenal stenosis

Development of the Midgut

1. It includes the distal part of duodenum, small intestines , large intestines till the distal third of transverse colon

2. Observe how the midgut increased rapidly in length forms a loop to the apex the vitelline duct which also open to the widely open umbilicus

3. Observe how the dorsal mesentery also elongates and passing through it from the aorta to the yolk sac, the vitelline arteries which fuse and form the superior mesenteric artery which is the blood supply to midgut and its derivatives

In the development of midgut loop

1. Study the physiological hernia in the umbilical cord

2. Study the development of cecum and appendix

3. Study the development of jejunum and ileum

4. Study how the midgut loop in the umbilical cord rotates around the axis (superior mesenteric artery and vitelline duct) a counter clock wise rotation 90 degrees

5. Later the midgut as it returns to the abdominal cavity it rotates counter clock wise an additional 180 degrees so the total rotation is 270 degrees counter clock wise

After rotation of the midgut observe the following results:

1. Transverse colon lies in front of superior and 2nd part of duodenal mesenteric artery

2. Third part of duodenum lies behind the superior mesenteric artery

3. Cecum and appendix comes into right iliac fossa so:

A. Ascending colon and B. B. right colic flexure are formed 4. Large gut after rotation lie laterally and encircle

the centrally placed small gut 5. Primitive mesentery of: a. Ascending colon and descending colon by

which they fuse with parietal peritoneum on the posterior abdominal wall become retroperitoneal organs

6. Primitive mesenteries of : a. Jejunum b. Ileum c. Transverse colon d. Sigmoid colon Persist as mesentery of small intestine,

transverse colon sigmoid mesocolon 7. As the midgut returns to the abdominal cavity,

the vitleline becomes obliterated and severs its connection with the gut

Study the gut rotation defects

1. Study the gut atresias and stenoses

2. Study the body wall defects:

a. Omphalocele: umbilical hernia through umbilical ring (physiological hernia)

It is covered by amnion

occurs in 2.5/10000 births

it has high rate of mortality

it has 50% with cardiac abnormalities

50% with chromosomal abnormalities

b. gastroschisis : hernia through the region of right umbilical vein

Development of hindgut

It includes the distal third of transverse colon descending and sigmoid colons and upper part of anal canal

Observe the following points:

1. The inferior mesenteric artery is the blood supply to the hindgut

2. The endoderm of the hindgut also forms the lining of bladder and artheria

3. The terminal portion of hindgut enters the post region of cloaca which forms the primitive anorectal canal

4. Cloaca is an endodermal lined cavity covered ventrally by surface ectoderm

5. Allantois enters into anterior portion of the cloaca forms urogenital sinus

6. Cloacal membrane lies between endoderm and ectoderm

7. Urorectal septum is an emerging mesoderm and ectoderm between allantois and hindgut

In development of hindgut notice:

1. At the end of 7th week the cloacal membrane is ruptured which creates the following:

A. Anal opening of hindgut

B. Ventral opening for urogenital sinus

C. Between the 2 forms perineal body

2. Observe that proliferation of ectoderm closes the most of caudal region of the anal canal then at ninth week of development the ectodermal part of anal canal

3. Pectinate line lies at the junction between the endoderm and ectoderm part of anal canal

Abnormalities in the development of hindgut:

Explain the following:

1. Different types of fistula :

a. Urorectal fistula

b. Rectovaginal fistula

2. Rectoanal atresia or fistula

3. Imperforated anus